Saturday, January 16, 2016

LIGO

Rumors are flying that LIGO has actually spotted gravitational waves already. As you probably know if you read the rumors, there's a team which generates and inserts fake events in the data stream to make sure the rest of the team can catch them--and they did once already. That's kind of rough on the collaboration--a couple of months of late night work verifying and then "Good job, you caught the fake." "For this I missed the kid's game?"

So this might be another calibration test. But it might not.

A team of radio astronomers think there's another black hole in our galaxy, and if their models are correct it would be an intermediate size one. If verified, this would be the first black hole found in this size class: O(100,000)x Sun's mass. The cloud it is supposedly embedded in is about 200 light years from SgA*, so we won't be seeing any fireworks anytime soon, but if these things are more common than previously expected, there will be more black hole mergers and a better chance for LIGO to see waves from the results.

If you are wondering how two black holes can wind up merging, there are two effects. The first is that each of them interacts with stars and gas as they orbit each other, and typically the result will be that the nearby star gets a kick in its speed and the heavy black hole slows down a bit. You can think of it as like a satellite's orbit decaying as it pushes through the air, except that the interaction isn't friction but gravitational scattering.

The second effect is the one LIGO is looking for: as the two get very close together they emit stronger and stronger gravitational waves. Those take away energy from the system, and the objects necessarily orbit more and more closely until they collide. As that happens there is supposed to be a characteristic "chirp" of very rapid gravitational waves, that LIGO is designed to spot. The pair (assuming the new object really is a black hole--they infer that from the very large spread of velocities of gas in the cloud) near us aren't going to collide anytime soon, though. I'm not sure I'd want a ring-side seat for the festivities. I think we'd be safe enough at this range.